Composite strip windable to form a helical pipe and method therefor

ABSTRACT

A composite strip  10  windable to form a helical pipe is disclosed. The composite strip  10  comprises: an elongate plastic strip  11  having a base portion  12  and at least one lengthwise extending rib portion  20  upstanding from the base portion  12 ; and an elongate reinforcing strip  30  extending lengthwise and supported laterally by the rib portion  20 , the reinforcing strip  30  having a height to thickness ratio of at least three to one and orientated substantially perpendicular to the base portion  12 . When wound into a helical pipe, the reinforcing strip  30  reinforces the pipe against radial crushing loads. A bead  40  seals the reinforcing strip  30  from the environment. Desirably the composite strip  10  further comprises: a planar lamina  50  extending lengthwise and bonded to the base portion  12 , the lamina  50  having a higher Young&#39;s modulus and strength than those of the plastic strip  11 . The lamina  50  greatly improves the pressure rating of a pipe wound from the strip  10.

FIELD OF THE INVENTION

This invention relates to improvements to reinforced ribbed structures,and in particular to reinforced or strengthened helically wound pipes ortubes made from a composite of materials.

BACKGROUND

It is well known that plastic pipes can be made by helically winding aplastic strip having a series of spaced apart upstanding ribs extendinglongitudinally of the strip, either at room temperature or at anelevated temperature where the plastic becomes more flexible. This formof helically wound tube is already well known in the piping industry andis described in patents by the applicant relating both to the form ofthe plastic strip and the form of the machine by means of which thepipes or tubes are produced from such strips.

For these pipes to perform in a high performance applications, in orderto attain the necessary degree of strength, the wall thickness of theplastic strip must be quite substantial, as well as that of the ribs.Alternatively the finished pipes or tubes can be reinforced withstrengthening or reinforcing members.

In applications where the reinforced tubes or pipes are buried in atrench or subjected to high earth loads, the strength of the pipe ortube is of extreme importance.

The applicant's Australian Patent No. 607431 discloses a method ofproducing a reinforced plastics tube utilising a reinforcing memberplaced between the ribs in such a manner that the deflection resistanceof the finished pipe or tube is materially increased. The reinforcingmember comprises a metal member having a profile of U-shapecross-section, the free ends of the reinforcing member being designed toengage beneath opposed flange formations of a pair of adjacent ribs tothereby lock the metal strip in position between the ribs and in turnstiffen the ribs and the finished pipe.

The applicant's Australian Patent No. 661047 discloses an improvementover the disclosure of Australian Patent No. 607431 referred to above.The improvement is provided by the provision of a reinforcing memberhaving a central body portion of inverted U or V-shaped cross-sectionthat has a radial height greater than the height of the ribs whereby theeffective external diameter of the composite pipe is substantiallyincreased. This provides a stiffer pipe.

Known helically wound composite pipes are formed in a multi-stageoperation. The plastic body is extruded and then is helically wound toform a pipe. Elongate steel reinforcing members are separatelyroll-formed into a profile providing the required stiffness (such as theinverted U or V-shaped profiles referred to above). The roll formedsteel profile is then rolled to a radius approximating that of thehelically wound plastics body. Finally, the profiled and radiusedreinforcing member or members are wound on to the outside of theplastics pipe to form a composite pipe of the requisite stiffness.

When using the reinforcing members disclosed in Australian Patent Nos607431 and 661047, the step of rolling the steel reinforcing member to aradius of approximating that of the plastics pipe involves straining thesteel reinforcing member beyond its elastic limit. This requires theapplication of considerable force during the rolling process. Incontrast, winding of the extruded plastics profile into a helical pipegenerally requires much less force due to the material properties of theplastics.

An object of the present invention is to provide certain improvements,beyond those disclosed in the aforesaid Patent Specifications 607431 and661047, to reinforced helically wound plastics tube or pipe, which areeffective to stiffen the tube or pipe through the addition ofreinforcing members formed of plastics material and/or of metal tothereby produce a composite plastics and metal structure.

It is another object of the invention to provide an improved compositestrip that can be wound formable into a helical pipe or tube without theneed for the addition of a reinforcing member during or after the pipewinding process. It is also an object of the invention to provide amethod of producing such a composite strip.

It is yet another object of the present invention to provide a method ofproducing a helically wound pipe that obviates the need for pre-rollingthe reinforcing member before it is introduced into the plastics body ofthe strip.

It is yet another object of the present invention to provide a helicallywound plastics pipe formed of two or more materials having differentcharacteristics so that the resultant or finished pipe or tube hasenhanced properties, and which can be produced at relatively low cost.

It is yet another object of the present invention to provide a helicallywound plastic pipe with a high pressure rating, that is, a pipe that iscapable of withstanding high internal pressure without failure.

SUMMARY OF THE INVENTION

According to a first aspect of the invention there is provided acomposite strip windable to form a helical pipe, the composite stripcomprising:

an elongate plastic strip having a base portion and at least onelengthwise extending rib portion upstanding from the base portion; and

an elongate reinforcing strip extending lengthwise and supportedlaterally by the rib portion, the reinforcing strip having a height tothickness ratio of at least three to one and orientated substantiallyperpendicular to the base portion,

wherein, when wound into a helical pipe, the reinforcing stripreinforces the pipe against radial crushing loads.

Preferably said height to thickness ratio is at least four to one.

Preferably the rib portion defines a lengthwise extending slot in whichthe reinforcing strip is retained, the reinforcing strip being laterallysupported by the walls of the slot.

Preferably the rib portion comprises a pair of parallel walls extendinglengthwise along the base portion, the parallel walls orientatedsubstantially perpendicular to the base portion.

Preferably the reinforcing strip is continuous and has a length which isco-extensive with the plastic strip.

Preferably the reinforcing strip is completely encapsulated so as toprevent exposure to the environment.

Preferably the plastic strip has an array of lengthwise extending slotforming rib portions spaced apart across the width of the strip, eachrib portion supporting an elongate reinforcing strip.

The composite strip may include various materials, however, preferablythe reinforcing strip is constructed from metal. More particularly,preferably the reinforcing strip is constructed from steel.

According to a second aspect of the invention there is provided acomposite strip windable to form a helical pipe, the composite stripcomprising:

an elongate plastic strip having a base portion and a lengthwiseextending rib portion upstanding from the base portion;

an elongate planar reinforcing strip extending lengthwise and supportedby the rib portion, the reinforcing strip having a height to thicknessratio of at least three to one and orientated substantiallyperpendicular to the base portion; and

a planar lamina extending lengthwise and bonded to the base portion, thelamina having a higher Young's modulus and strength than those of theplastic strip,

wherein, when wound into a helical pipe, the reinforcing stripreinforces the pipe against radial crushing loads and the laminaimproves the pressure rating of the pipe.

Preferably said height to thickness ratio is at least four to one.

According to a third aspect of the invention there is provided ahelically wound composite pipe produced from a composite strip, thecomposite strip comprising:

an elongate plastic strip having a base portion that forms the inside ofthe wound pipe and a lengthwise extending rib portion upstanding fromthe base portion; and

an elongate metal reinforcing strip extending lengthwise and supportedby the rib portion, the reinforcing strip having a height to thicknessratio of at least three to one and orientated substantiallyperpendicular to the flat side of the base portion,

wherein the orientation of the reinforcing strip with respect to thebase portion remains substantially unchanged after the winding of thestrip to form the pipe.

Preferably said height to thickness ratio is at least four to one.

Preferably the reinforcing strip is continuous and has a length which isco-extensive with the pipe.

Preferably the reinforcing strip is constructed from metal. Preferablythe metal is steel.

Desirably the composite strip further comprises:

-   -   a planar lamina extending lengthwise and bonded to the base        portion, the lamina having a higher Young's modulus and strength        than those of the plastic strip.

According to a fourth aspect of the invention there is provided a methodof producing a helically wound steel reinforced plastics pipe comprisingthe steps of:

extruding a plastics profile having a base portion and a lengthwiseextending rib portion upstanding from the base portion;

introducing an elongate straight edged reinforcing strip so as to extendlengthwise of the rib portion and supported laterally thereby, the metalstrip having a height to thickness ratio of at least three to one andorientated substantially perpendicular to the flat side of the baseportion, thereby producing a straight composite strip;

helically winding the composite strip; and

inter-engaging the adjacent edges of adjacent convolutions of the stripso as to form a helical pipe.

Preferably the rib portion is extruded to define a lengthwise extendingslot shaped to receive and support the reinforcing strip.

Preferably the rib portion is extruded to comprise a pair of parallelwalls extending lengthwise along the base portion, the walls orientatedsubstantially perpendicular to the flat side.

Preferably the method further comprises the step of encapsulating thereinforcing strip.

Preferably the extruding and introducing steps occur together in across-head extrusion die.

According to a fifth aspect of the invention there is provided a methodof producing a helically wound steel reinforced plastics pipe comprisingthe steps of:

extruding a plastics profile having a base portion and a lengthwiseextending rib portion upstanding from the base portion; and

introducing an elongate metal reinforcing strip into the rib portion,the metal strip having a height to thickness ratio of at least three toone and orientated substantially perpendicular to the base portion,thereby producing a composite strip;

bonding a lamina to the flat side of the base portion, the lamina havinga higher Young's modulus and strength than those of plastic strip; and

helically winding the composite strip; and

inter-engaging the adjacent edges of adjacent convolutions of the stripso as to form a helical pipe.

Preferably the method has further steps between the bonding andhelically winding steps, the further steps comprising:

directing the straight composite step to a spool having a hub rotatingabout a substantially horizontal axis, with the base portion of thestrip facing the underside of the hub;

driving the spool so as to pull the straight composite strip towards thespool and so as to wind the strip around the hub of the spool from itsunderside;

transporting the spool to a site; and

unwinding the strip from the spool.

DETAIL DESCRIPTION OF PREFERRED EMBODIMENTS OF THE INVENTION

Several preferred embodiments of the invention are illustrated in theaccompanying representations in which:

FIG. 1 shows a cross-sectional view of a composite strip according to afirst embodiment of the invention.

FIG. 2 shows an exploded view of the strip of FIG. 1.

FIG. 3 is a perspective view of the composite strip shown in FIG. 1.

FIG. 4 is a perspective view of a helically wound composite pipe woundfrom the profile shown in FIGS. 1 and 3.

FIG. 5 shows a part-sectional view of the pipe of FIG. 4 revealing thereinforcing element.

FIG. 6 is a perspective view showing the reinforcing element beingintroduced to the profile.

FIG. 7 shows a cross sectional view of a composite strip according to asecond embodiment of the invention.

FIG. 8 shows a cross sectional view of adjacent convolutions of acomposite strip according to a third embodiment of the invention.

FIG. 9 shows a cross sectional view of a composite strip according to afourth embodiment of the invention.

FIG. 10 shows a cross sectional view of a composite strip according to afifth embodiment of the invention.

FIG. 11 shows a cross sectional view of a composite strip according to asixth embodiment of the invention.

FIG. 12 shows a cross sectional view of a composite strip according to aseventh embodiment of the invention.

FIG. 13 shows a cross sectional view of a composite strip according to aeighth embodiment of the invention.

FIG. 14 shows a perspective view of a spool drive assembly for use withembodiments of the invention.

Referring to FIGS. 1 and 2, an elongate composite strip 10 that iswindable to form a helical pipe is shown. The composite strip 10comprises an elongate plastic strip 11 and an elongate metal reinforcingstrip 30. The plastic used for this embodiment of the invention ispolyethylene although other suitable plastics may be used.

The plastic strip 11 has a base portion 12 with a substantially flatside 14. A plurality of length-wise extending rib portions 20 projectsupward from the base portion 12. In this embodiment, each rib portion 20comprises a pair of parallel walls 22 and 24 that extend length-wisealong the base portion 12 to define a length wise extending slot 23. Theslot 23 is sized and shaped to snugly receive the reinforcing strip 30as shown best in FIG. 2.

A bead of plastics 40 is positioned to bridge the gap between the upperends of the rib walls 22 and 24 and thereby en-capsulate the reinforcingstrip 30 completely. This prevents exposure of the reinforcing strip 30to the environment and therefore assists in preventing corrosion.

In the first embodiment of the invention, an array of three lengthwiseextending rib portions 20 spaced apart across the width of the strip areprovided. Each rib portion 20 supports a corresponding elongate planarmetal reinforcing strip 30. In other embodiments of the invention, moreor less ribs and reinforcing strips may be used. The ribs 20 thatsupport the elongate metal reinforcing strips 30 need not be continuous.The ribs 20 can be any shape provided they support the verticallyoriented reinforcing strips 30.

Referring to FIG. 4, a helically wound composite pipe produced byhelically winding the composite strip shown in FIGS. 1, 2 and 3 isshown. The joint between adjacent edges 18 and 16 of adjacentconvolutions of the strip is best seen in cross-section in FIG. 1.

Comparing FIGS. 1 and 4, it is apparent that the orientation of thereinforcing strips 30 with respect to the flat side 14 of the baseportion 12 remains substantially unchanged after the winding of thestrip to form the pipe. The rib portions 20 provide support for thereinforcing strips 30 particularly during the winding of the strip 10.During winding of the strip 10 to form a helical pipe, the reinforcingstrips 30 are bent about an axis substantially transverse to the strip10. This causes plastic deformation of the reinforcing strips 30. Therib portions 20 assist in preventing the reinforcing strips 30collapsing sideways and towards the base of the plastic strip 12.

FIG. 5 shows an arc-shaped portion of a reinforcing member 30 after ithas been bent to wind the pipe shown in FIG. 4. Small areas of buckling32 are illustrated.

It is important that these areas of buckling are either non-existent orrelatively small. If excessive buckling is present, the ability of thepipe to withstand radial crushing loads is compromised.

It is also important to keep the mass of the profile to a minimum whileat the same time maintaining the performance criteria to ensure materialcosts are minimised.

The dimensions and shapes of the plastic strip 12 and the elongate metalreinforcing strips 30 can be varied to suit the diameter of the pipe tobe wound. The below table shows a range of configurations suitable forpipes of internal diameter ranging from 300 to 600 millimetres.

Internal Steel Height to No. Diameter Thickness Steel Height Thicknessof Steel Steel mm Mm mm Ratio bands Material 150 0.6 4 6.7:1  3 CA3 SNG300 0.6 12 20:1 3 CA3 SNG 375 0.6 12 20:1 3 CA3 SNG 450 0.6 14 23:1 3CA3 SNG 525 0.6 16 27:1 3 CA3 SNG 600 0.8 16 20:1 3 CA3 SNG 675 1.0 1616:1 3 CA3 SNG 750 1.2 16 13:1 3 CA3 SNG 825 1.6 16 10:1 3 CA3 SNG 9001.6 16 10:1 3 CA3 SNG 1050 1 19 19:1 3 CA3 SNG 1200 1.2 19 16:1 3 CA3SNG CA3 SNG: Uncoated cold rolled mild steel.

The height, thickness and number of steel reinforcing bands used arevariables that influence the stiffness of the wound pipe. With pipes oflarger diameter, the contribution of the plastic to the stiffness of thepipe is relatively small (<10%). With pipes of smaller diameter thecontribution of the plastic to the stiffness of the pipe is higher(approximately 30% for a pipe having internal diameter of 300 mm).

The height to thickness ratio of the reinforcing strips 30 are importantfor a number of reasons. Reinforcing strips having a high height tothickness ratio are preferable from the point of view of pipe stiffnessand efficient use of material but this must be weighed against theinstability that may result. Instability may cause the reinforcingstrips 30 to collapse sideways towards the base of the plastic strip 12or may cause excessive buckling (buckling is illustrated in FIG. 5).

Selection of a steel with the optimum Young's modulus (or tensilemodulus) and yield strength for this application is also important.Where yield strength is excessive, buckling is more likely.

With the range of profiles described in the above table, and with a ribportion thickness range of 1.4 to 1.8 mm, pipes can be wound that arestable of relatively low weight and have excellent resistance to radialcrushing loads.

Although the embodiment described above uses steel reinforcing, elongateplanar reinforcing strips constructed from other materials may be used.

The addition of the reinforcing strips 30 to the plastic strip 12 canalso assist in improving the pressure rating of the pipe. The compositestrips described above can further incorporate other elements to improvethe pressure rating of the wound pipe. For instance, lamina of fibrefabric (eg glass fibre), plastic or steel may be provided to improve thepressure rating of the pipe. Any material having a Young's modulus andstrength that exceeds that of the plastics material of the strip can beused. The lamina may be incorporated into the profile (strip 12) in anysuitable way. For instance, the lamina may be welded to the base of thestrip 12 or may be cross-head extruded into the base of the strip 12.

Improved interlocking edge features may also be provided to enhance thepressure rating of the pipe. Examples of profiles constructed forhigh-pressure applications are shown in FIGS. 7 through to 13.

Referring to FIG. 7, a second embodiment of the invention is shown wherethe composite strip 10 is extruded from PVC. A mechanical lock isprovided by a male edge member 16 and a female edge member 18 formedfrom the plastic strip 11. Reinforcing strips 30 of the type describedabove are also provided. This profile is cross head extrudedencapsulating the reinforcing strips 30 as the composite strip 10 isproduced obviating the need to add a sealing bead as previouslydescribed. A lamina 50 is incorporated into the base portion of thestrip 11. The lamina 50 has a higher Young's modulus and strength thanthe PVC plastic strip 11. When wound into a helical pipe, this profilecan provide a high pressure pipe suitable for conveying fluids underpressure. Although adjacent convolutions are not directly boundtogether, the thickness of the plastic and design of the mechanical lockformed by adjacent edges 16 and 18 ensures that the pipe is able towithstand significant internal pressures.

FIG. 8 shows a cross sectional view of two adjacent convolutions ofcomposite strip 10 according to a third embodiment of the invention.This composite strip 10 comprises a polyethylene extruded strip 11having three rib portions 20 extending from a base portion 12, each ribportion 20 supporting a reinforcing strip 30. A fourth rib portion 21supporting a fourth reinforcing member 31 is also provided. The locationof the fourth rib 21 and reinforcing strip 31 is at the edge of theprofile to strengthen the wound pipe along the gap between the lamina ofadjacent convolutions. This gap 54, is shown in FIG. 8.

By providing reinforcement on top of the lock between adjacentconvolutions of the composite strip and over the region where the laminais discontinuous, a pipe capable of withstanding high pressure can beproduced.

A fourth embodiment of the invention is shown in FIG. 9. This embodimentof the invention is similar to the third embodiment of the inventionexcept instead of providing an additional rib and reinforcing memberover the joint area, the female lock section has a thick wall to providethe pressure capacity where the lamina is discontinuous.

A fifth embodiment of the invention is shown in FIG. 10 where noadditional features are provided between adjacent convolutions to coverthe area in which the laminae are discontinuous.

A sixth embodiment of the invention is shown in FIG. 11. With thisembodiment of the invention, an additional lamina is welded to the edgesection of the profile as illustrated.

A seventh embodiment of the invention is shown in FIG. 12. Thisembodiment of the invention differs slightly to the above-describedembodiment in that the additional lamina 55 is inserted during the pipewinding process.

A final embodiment of the invention is shown in FIG. 13. With thisembodiment of the invention, a continuous lamina is either cross headextruded in the profile base 12 and edge locking areas or is welded tothe base after extrusion.

Other embodiments of the invention may be provided with the laminaeither bonded to the base of the strip 12 or embedded within the base ofthe strip 12.

Materials having directional properties may be used as or within thelamina. For instance, orientated plastic film strips that are strong ina longitudinal direction and weak in a transverse direction may be used.Such strips may improve the “hoop” strength of the wound pipe.

Plastic film strips that are strong in a transverse direction and weakin a longitudinal direction may also be used.

In some applications it will be desirable to form a lamina from two (ormore) plastic film strips that are strong in mutually orthogonaldirections thereby resulting in a composite of high strength in alldirections.

Examples of suitable materials having directional properties includehighly stretched polyolefin sheet. Such sheets have a high proportion ofmolecules orientated in the same direction which provides a high Young'smodulus and yield strength.

Currently, helically wound composite pipes are formed in multi-stageoperations. Generally a plastics body is extruded within a factoryenvironment and is then wound onto a spool for transport. The extrudedstrip is then unwound from the spool and passed through a windingmachine that can also be located within a factory or alternatively canbe located at the site where the final pipe is required. Finally,elongate steel reinforcing strips are rolled onto the newly wound pipe.In many applications, the steel reinforcing strips are pre-rolled to aradius approximating that of the helically wound plastics body beforethey are introduced onto the outside of the plastics pipe to form acomposite pipe with a requisite stiffness. The pre-bending of thereinforcing strip is required where reinforcing members have a highdegree of stiffness through the relevant bending axis.

The process of forming a helical pipe from the profile described abovewith reference to FIGS. 1, 2, 3, 5 and 6 is simplified since thereinforcing members 30 are introduced into the strip at an early stageof manufacture and before the pipe is wound.

A method of constructing a composite strip 10 windable to form a helicalpipe is shown in FIG. 6. A plastics strip 11 is extruded having asubstantially flat sided base portion and an array of parallel spacedapart length wise extending rib portions 20 upstanding from the baseportion 12. Next a plurality of elongate metal reinforcing strips 30 areintroduced the rib portions 20. The reinforcing strip 30 has a height tothickness ratio of at least four to one and is orientated substantiallyperpendicular to the flat side 14 of the base portion 12.

The introducing or inserting step described above occurs while theplastic strip is lying substantially flat. The reinforcing strips 30 areinserted straight without any pre-bending. Finally beads of plastic 40(as shown in FIGS. 1 and 2) are extruded onto the tops of the ribportions 20 to encapsulate the reinforcing strips 30.

A further method of constructing a composite strip windable to form ahelical pipe is as follows. Plastics material and steel strip areintroduced into an extrusion cross-head die where the two materials areintegrated into one composite profile, such as the composite stripdescribed above and shown in FIG. 3. A composite strip formed bycross-head extrusion may differ slightly from the profile describedabove in that the beads of plastic 40 (as shown in FIGS. 1 and 2) wouldnot be required—instead, the cross-head extrusion die could be designedsuch that the steel strip exits the die completely encapsulated withplastics material.

Having produced a composite reinforced strip, it is possible to directlywind that strip into a helically wound pipe such as the pipe shown inFIG. 4 or alternatively, the strip can be rolled onto a spool for lateruse.

The ability to roll the composite profile onto a spool for transportprovides a number of advantages. For instance, a single spool can betransported to the field and positioned adjacent a pipe winding machinelocated where the final pipe is required. Composite pipe can then behelically wound in a single operation without the need for large amountsof specialised equipment.

In order to be able to spool the straight composite strip 12 withut thesteel reinforcing strips 30 buckling it was necessary to develop a newmethod of spooling. Existing conventional spooling methods create astrip path that reverse bends the strip and then straightens it prior tothe strip going onto the hub of the spool. The spool is rotated about ahorizontal axis with the strip being fed to the top or upperside of thespool. For plastic strips without steel this method is satisfactory.However, when there is steel reinforcing in the strip, this method isnot suitable as it causes the steel reinforcing 30 to buckle.

FIG. 14 shows a spool drive assembly 100 developed for spooling steelreinforced composite strip 10. The spool 101 is supported for rotationabout a horizontal axis 102. A strip guide 110 is provided to distributethe strip 10 across the width of the spool's hub. An endless pneumaticcylinder 114 driving on a rod 112 drives the strip guide 110 back andforth.

The spooling method developed for steel reinforced strip and shown inFIG. 14 has a strip path that minimises any load applied to the stripthat could cause buckling. The strip path onto the spool 101 with thisarrangement is a straight path to the bottom or underside 103 of thespool with the ribs facing down and hence the base portion 12 facing upallowing the strip to be bent in the correct orientation on the spool(ribs facing outwards, as they do in the wound pipe).

The method of controlling the speed of rotation of the spool 101developed for this new method relies on the tension in the strip 10(torque on the motor). In addition to changing the spooling methods, theoptimum spool hub size needs to be selected to prevent the ribs bucklingduring the spooling process. An initial hub size of 450 mm was trialedwhich was suitable for some of the steel thicknesses, however as thesteel becomes thicker and taller the hub size needs to be increased. Forthe current strip 10 made for pipes up to 750 mm in diameter, a hub sizeof 1000 mm is required.

The profiles of the second to eighth embodiments of the invention asillustrated in FIGS. 7 to 13, can be constructed using the methoddescribed above for the profile of the first embodiment of the inventionas shown in FIGS. 1 to 6. The lamina can be introduced in a separatestep after the strip has been extruded.

While the present invention has been described in terms of a preferredembodiment in order to facilitate better understanding of the invention,it should be appreciated that various modifications can be made withoutdeparting from the principles of the invention. Therefore, the inventionshould be understood to include all such modifications within its scope.

1. A helically wound composite pipe produced from a composite strip, thecomposite strip comprising: an elongate plastic strip having a baseportion that forms the inside of the wound pipe and a lengthwiseextending rib portion upstanding from the base portion; an elongatereinforcing strip extending adjacent to or integral with the ribportion; and an elongate planar lamina extending lengthwise and bondedto the base portion, the lamina having a higher Young's modulus andstrength than those of the plastic strip.
 2. A pipe as claimed in claim1 wherein adjacent wound convolutions of the lamina of the pipe are notdirectly bonded together.
 3. A pipe as claimed in claim 2 wherein thelamina comprises fibre fabric.
 4. A pipe as claimed in claim 3 whereinthe fibre fabric comprises glass fibre.
 5. A pipe as claimed in claim 4wherein the rib portion comprises a pair of parallel walls extendinglengthwise along the base portion, the rib portion defining a lengthwiseextending slot in which the reinforcing strip is retained, the stripbeing laterally supported by the walls of the slot.
 6. A pipe as claimedin claim 5 wherein the walls are orientated substantially perpendicularto the base portion.
 7. A pipe as claimed in claim 6 wherein thereinforcing strip is continuous and has a length which is co-extensivewith the plastic strip.
 8. A pipe as claimed in claim 7 wherein thereinforcing strip is completely encapsulated so as to prevent exposureto the environment.
 9. A pipe as claimed in claim 8 wherein the plasticstrip has an array of lengthwise extending slot forming rib portionsspaced apart across the width of the strip, each rib portion supportingan elongate reinforcing strip.
 10. A pipe as claimed in claim 9 whereinthe reinforcing strip is constructed from metal.
 11. A pipe as claimedin claim 10 wherein the reinforcing strip is constructed from steel. 12.A method of producing a helically wound steel reinforced plastics pipecomprising the steps of: extruding a plastics profile having a baseportion and a lengthwise extending rib portion upstanding from the baseportion; extruding a lamina into the base portion, the lamina having ahigher Young's modulus and strength than those of plastic strip;introducing an elongate metal reinforcing strip into the rib portion,the metal strip having a height to thickness ratio of at least three toone and orientated substantially perpendicular to the base portion,thereby producing a composite strip; helically winding the compositestrip; and inter-engaging the adjacent edges of adjacent convolutions ofthe strip so as to form a helical pipe.
 13. A method as claimed in claim12 wherein the steps of extruding are cross-head extruding.
 14. A methodas claimed in claim 13 wherein the lamina comprises fibre fabric.
 15. Amethod as claimed in claim 14 wherein the fibre fabric comprises glassfibre.
 16. A method as claimed in claim 15 wherein the rib portion isextruded to define a lengthwise extending slot shaped to support thereinforcing strip.
 17. A method as claimed in claim 16 wherein the ribportion is extruded to comprise a pair of parallel walls extendinglengthwise along the base portion, the walls orientated substantiallyperpendicular to the base.